The invention relates to solar collectors and more particularly to solar collectors having substantially increased energy rejection during stagnation to facilitate the use of structural materials incapable of withstanding the high temperatures usually encountered in conventional solar collectors during stagnation.
A typical stagnating single glazed selective absorber collector will exceed 400.degree. F. during stagnation. This requires that a typical collector be constructed of materials capable of withstanding at least such temperatures for extended periods of time during the down time or stagnation periods which typically occur during the summer months. Materials capable of withstanding such temperatures over a period of years in which a collector will be in use are much higher cost than materials capable of withstanding, for example, temperatures up to only about 180.degree. F. for like periods of time.
There are, of course, many types of active solar collectors in use at present. Those supplying heat in the medium temperature range, i.e., temperature ranges between about 100.degree. F. and about 250.degree. F., utilize one or more cover sheets or glazing. While this glazing enhances the performance of such collectors, it also creates a problem in materials in that the glazings, the absorber, fluids flowing through the collector, insulation within the collector, and sealants used in constructing the collector all must be able to withstand increased temperatures during no-flow or stagnation conditions or periods. Since no energy is being extracted during such periods, the energy loss from the collector must equal the solar gain. This results in extremely high temperatures being reached within the collector, necessitating the use of expensive high temperature enduring collector materials. Such materials and components could comprise, for example, a glass or fluorocarbon film glazing, black chrome selective absorber, silicone or paraffinic oil heat transfer fluids, silicone, or polyimide foam or fiberglass with high temperature binders for insulations, and silicone, fluorocarbon or other high temperature sealants.
In the case of a selective absorber collector in accordance with the invention, energy is conducted from the low emittance selective absorber to the high emittance glazing and radiated and convected to the atmosphere like an unglazed collector, thereby maintaining low stagnation temperatures.